    Non-patent document 1: AMD Application Note Vol. 1 December, 2002 p. 1 to P. 19 Alkali Metal Dispenser for OLED    Panted document 1: Japanese Patent Laid-open Publication No. 2001-6878, THIN FILM EL ELEMENT AND ITS DRIVING METHOD
Various display devices characterized by its thin and flat structure such as a liquid-crystal display and a plasma display are now prevailing widely as a display device in place of a cathode-ray tube. Also, a display using an organic EL expected to be a main stream of the next-generation display is recently studied and developed. Since the organic EL converts electricity into light using electroluminescence, it hardly generates heat and uses less power. Also, it has a characteristic that a sharp image can be displayed regardless of the viewing angle, unlike the liquid-crystal display.
FIG. 8 is a sectional view of a conventional light-emitting element. The conventional light-emitting element is formed by sequentially laminating an anode 102, a positive-hole injection layer 103, a positive-hole transport layer 104, a light-emitting layer 105, an electron transport layer 106, an electron injection layer 107 and a cathode 108 on a glass substrate 101. For the anode, a transparent electrode material having a large work function, ITO (indium Tin Oxide) is used, while aluminum with a small work function is used for the cathode. As the light-emitting layer 105, a low molecular organic EL such as Alq3 and NPB or a high molecular organic EL such as PPV and poly (3-alkylthiophene) is used. In order to improve bonding performance of the light-emitting layer between the anode and the cathode made of an inorganic material and the light-emitting layer made of an organic material, a positive-hole transport layer and a positive-hole injection layer are normally interposed between the light-emitting layer and the anode, and the light-emitting element is made in the multilayered structure by interposing an electron transport layer and an electron injection layer between the light-emitting layer and the cathode. Conventionally, as a material for the positive-hole transport layer or the positive-hole injection layer, an organic material such as TPD, PEDOT is used, while as the material for the electron transport layer or the electron injection layer, an organic material doped with alkali metal is used. By doping the organic layer on the cathode interface with alkali metal, a radical anion of the organic molecule is generated and it acts as an internal carrier at application of an electric field, which can reduce driving voltage of the organic EL (non-patent document 1).
However, there is a problem that in the organic material doped with alkali metal used as a material for the electron transport layer and the electron injection layer, the alkali metal has high reactivity and is likely to be formed into a hydroxide. Therefore, in order to prevent the organic material doped with alkali metal or formed thin film from reacting to moisture in the atmosphere or other impurities in the manufacturing process of a light-emitting element, there are problems that process control should be performed strictly, and sealing of the light-emitting element or the light-emitting device should be complete to prevent external air from leaking to the formed light-emitting element and reacting with steam. Also, it is difficult to form a light-emitting element with sufficiently long life.
As an electron transport layer, an EL element using a material including empty fullerene such as C50, C70 is known (patent document 1). However, the empty fullerene has a problem that electron affinity is relatively small, and electron injection efficiency is poor, and therefore, light-emitting efficiency is not high.